Immunoassays, and in particular techniques such as the radioimmunoassay and enzyme immunoassay, have revolutionised diagnostic medicine. Such assays are based upon the detection of antibody-antigen interactions. Numerous detection systems are available, including the use of enzyme labelled antigens, antibodies or complexes thereof, which require incubation with specific substrates in order to measure the end-point either colour metrically or flourescently.
Whilst these assays are sensitive, the detection systems are complex, and therefore expensive. Typically, the detection systems require several washing steps, meaning that the assays are generally unsuitable for point-of-care type assessment.
Agglutination immunoassays are well known in the art, and rely upon agglutination of particles to which an antigen or antibody is bound to indicate the presence of the corresponding antigen or antibody in a sample. In one of the simpler forms of an agglutination assay, antibodies to a particular analyte are bound to a bead or other visible material. For example, in the latex agglutination reaction, the presence of analyte present in the system is detected, bound to the surface of the latex beads. Typically, the antibody will be divalent, thus causing the latex beads to form clumps in the presence of an analyte. Such clumps indicate a positive result, and can be seen with the naked eye.
Bio-Diagnostics Ltd, in their patent application No. WO 99/35497, disclose a agglutination based assay device for identifying blood groups. The device incorporates four blood group testing channels, namely three capillary channels which part way along their lengths are formed into one or other of the indicators A, B, or O and a further capillary channel which incorporates an indicator in the form of a + symbol, indicative of rhesus status. Upstream of each indicator, the channels have an agglutination system which causes blood corresponding to the indicator of the channel to agglutinate and impede flow in the channel. Thus, when a sample is introduced, blood will flow unhindered along the capillary channels, and will cause the indicators to be coloured red. Where the blood is of Group A, for example, an agglutination will take place, preventing blood from flowing into the indicator portion of the channel. Thus, the indicator will not become coloured red, but will remain white. In this way, the blood group can be determined visually. However, there is room for human error in such a test, due to reliance on the user to correctly read and interpret the visual result. The assay is also reliant upon the agglutination causing a total blocking of the capillary, therefore erroneous results may arise if the agglutination reaction is weak or if the sample volume is insufficient to fill all the capillaries. Also, the result may be unstable, yielding a different result if read at a later time.
WO04/83859 describes a capillary based agglutination assay, comprising a capillary pathway which contains a reagent system capable of causing agglutination with the analyte. The reagent system comprises an antibody bound to either the capillary walls at a predetermined location, or antibody bound to beads which are placed in the capillary system at a predetermined location. Upon application to the capillary pathway, a sample (and optionally ancillary reagents) flows along the pathway until it reaches the agglutination reagent system. If analyte is present, agglutination will occur, preventing or slowing further flow of the sample along the capillary. Detection means for the presence of the sample at the downstream end of the pathway are effected after a predetermined time from application of the sample—if no sample can be detected, then analyte is present, indicating a positive result. This device uses latex beads as the agglutination means.
The present invention aims to provide an improved agglutination based assay system.